(1) Field of the Invention
This invention relates to a method for fabricating a Group III nitride film, an underlayer for fabricating a Group III nitride film and a method for fabricating the same underlayer, particularly usable for semiconductor films constituting a light-emitting diode or a high velocity IC chip.
(2) Related Art Statement
Group III nitride films are employed as semiconductor films constituting light-emitting diodes, and recently, have received much attention as semiconductor films constituting high velocity IC chips for use in cellular phones.
Such Group III nitride films are usually fabricated by MOCVD methods. Concretely, a substrate on which Group III nitride films are formed is set onto a susceptor installed in a given reactor and then heated to 1000xc2x0 C. or over with a heater provided in or out of the susceptor. Thereafter, raw material gases are introduced with a carrier gas into the reactor and supplied onto the substrate.
On the substrate, the raw material gases are dissolved through thermochemical reaction into constituent elements, which are reacted to deposit and fabricate a desired Group III nitride film on the substrate.
There are few suitable substrates which have lattice constants and thermal expansion coefficients nearly equal to those of Group III nitride films. In this case, the difference in the lattice constants between the Group III nitride film and the substrate may induce more misfit dislocations at the boundary between the Group III nitride film and the substrate. In order to reduce such misfit dislocations, generally, a lower crystallinity buffer layer, which is fabricated at a lower temperature, is provided between the substrate and the Group III nitride film. In this case, the difference in lattice constant is compensated, and thus, many misfit dislocations are not created.
However, the misfit dislocations are not reduced sufficiently even though the buffer layer is provided, so that many dislocations of about 109-1010/cm2 may be created in the Group III nitride film, originating from the propagation of the misfit dislocations. As a result, the crystal quality of the Group III nitride film is degraded, and thus, the electrical and optical properties of the Group III nitride film are degraded.
In order to solve the above problem, attempts have been made to form a patterned mask made of SiO2 on the substrate and epitaxially grow the Group III nitride film laterally on the substrate. In this case, misfit dislocations are propagated laterally on the mask, and not propagated vertically. Therefore, the dislocation density of the Group III nitride film can be lowered in the region above the patterned mask.
However, since photolithography processes including etching steps are required in forming patterned masks, the total number of steps in the fabricating process for the Group III nitride film is increased and complicated.
It is an object of the present invention to provide a method able to easily fabricate a Group III nitride film having a lower dislocation density.
In order to achieve the above object, this invention relates to a method for fabricating a Group III nitride film, including the steps of preparing a given substrate, forming an underfilm with a concave-convex (i.e., a rough contoured) surface structure, in which 50% or below of the surface structure is occupied by a flat region. The underfilm is made of a first Group III nitride including 50 atomic percentage or over of elemental Al with respect to all of the Group III elements constituting the first Group III nitride, and another Group III nitride film is formed on the underfilm.
The inventors intensely worked to obtain a Group III nitride film having lower dislocation density using an easy fabricating process without a patterned mask made of SiO2. As a result, they found out that when the underfilm mentioned above is formed on a substrate and a given Group III nitride film is formed on the underfilm by a MOCVD method, the dislocation density of the Group III nitride film is reduced.
Such an underfilm can be made as follows. First of all, a Group V raw material gas and a Group III raw material gas are supplied onto a substrate at a flow ratio (Group V raw material gas/Group III raw material gas) of 600 or below, to form, on the substrate, a first film made of a Group III nitride including 50 atomic percentages or over of elemental Al for all of the Group III elements through the epitaxial growth for a given period. Then, the Group V raw material gas and the Group III material gas are supplied onto the substrate at flow ratio (Group V raw material gas/Group III raw material gas) of more than 600, to form, on the first film, a second film made of another Group III nitride including 50 atomic percentages or over of elemental Al with respect to all of the Group III elements through the epitaxial growth for a given period.
Moreover, a Group V raw material gas and a Group III raw material gas are supplied onto the substrate under a pressure of 20 Torr or below, to form, on the substrate, a first film made of a Group III nitride including 50 atomic percentage or over of elemental Al with respect to all of the Group III element through epitaxial growth for a given period. Then, the Group V raw material gas and the Group III material gas are supplied onto the substrate under a pressure of more than 20 Torr, to form, on the first film, a second film made of another Group III nitride including 50 atomic percentage or over of elemental Al with respect to all of the Group III elements through epitaxial growth for a given period.
Furthermore, a Group V raw material gas and a Group III raw material gas are supplied onto the substrate heated at 1100xc2x0 C. or over, to form, on the substrate, a first film made of a Group III nitride including 50 atomic percentage or over of elemental Al with respect to all of the Group III elements through epitaxial growth for a given period. Then, the Group V raw material gas and the Group III material gas are supplied onto the substrate heated at a temperature less than 100xc2x0 C., to form, on the first film, a second film made of another Group III nitride including 50 atomic percentage or over of elemental Al with respect to all of the Group III elements through epitaxial growth for a given period.
That is, the above-mentioned underfilm is stepwisely formed using two respective MOCVD fabricating conditions. In this case, the desired crystallinity of the underfilm is developed and a concave-convex structure is formed on the underfilm. If 50 atomic percentage or over of elemental Al with respect to all of the Group III elements is incorporated into the underfilm, the concave-convex contoured structure can be easily formed. Therefore, without using a complicated photolithography process, the underfilm can be easily formed in the same apparatus as that of the Group III nitride film.
Therefore, the Group III nitride film having lower dislocation density and good crystallinity can be fabricated without complicating the fabricating process.
Herein, the first through the third fabricating methods may be employed in isolation or in a combination of two or more. Moreover, another fabricating method may be employed. For example, if the flow ratio of (Group V raw material gas/Group III raw material gas) is set to 1000 or over, the underfilm can be formed in one step by using a single MOCVD fabricating condition.